The present invention relates to a fuel cell using a molten carbonate, such as alkali metal carbonate or alkali earth carbonate, as an electrolyte, to improve the passage of fuel and oxidant gases.
In a fuel cell of this type (hereinafter called "molten carbonate fuel cell"), electrode reactions occur when the electrolyte (i.e., a carbonate) is molten at high temperatures. Electrode reactions then occur more readily than in a phosphoric acid fuel cell or a solid electrolyte fuel cell. Hence, the molten carbonate fuel cell has a high power-generating efficiency, and requires no expensive metallic catalysts.
This fuel cell comprises a number of unit cells stacked one above another, and conductive separator plates each interposed between two adjacent unit cells. Many unit cells, each generating a low electromotive force of 1 V, are used and stacked to provide a greater electromotive force. Each unit cell has a porous anode electrode, a porous cathode electrode, and an electrolyte tile interposed between these electrodes. Each separator plate interposed between two unit cells electrically connects these unit cells, and has fuel gas passages for guiding a fuel gas (e.g., H.sub.2 or CO) to the anode electrodes, and also oxidant gas passages for guiding an oxidant gas to the cathode electrodes.
The separator plates used at present can be classified into three types, in accordance with the positional relation between the fuel gas passages and oxidant gas passages, and with the directions in which both gases flow. The first is the so-called cross flow type in which the fuel gas passages extend at right angles to the oxidant gas passages. The second is the so-called co-flow type wherein all gas passages extend in parallel, and the fuel gas and oxidant gas flow in the same direction. The third is the so-called counterflow type wherein all gas passages extend in parallel, but the fuel gas and oxidant gas flow in opposite directions.
When the fuel gas and oxidant gas are continuously supplied to the anode electrode and cathode electrode of each unit cell, the following electrode reactions occur: In the anode electrode, EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.CO.sub.2 +H.sub.2 O+2e (1) EQU (CO+H.sub.2 O+.fwdarw.CO.sub.2 +H.sub.2) (2)
In the cathode electrode, EQU 1/20.sub.2 +CO.sub.2 +2e.fwdarw.CO.sub.3.sup.2- ( 3)
As these reactions proceed in the unit cells, the fuel cell generates electrical energy. As can be seen from formula (2), CO supplied as fuel gas does not react with the anode electrode, but reacts with H.sub.2, thereby generating H.sub.2. H.sub.2, thus generated, reacts with the anode electrode, as according to formula (1).
It is desirable that the electrode reactions proceed uniformly on the entire electrode surfaces, so that the fuel cell can function reliably for a long period of time. However, the current density at the inlet of each of the fuel gas passages is twice as high as that at the outlet of the fuel gas channel, regardless of the type of separator plate used. Therefore, the electrical power becomes concentrated at the inlet portion of each fuel gas passages, or at the first one-third of the passages. For the same reason, heat generated by the electrode reactions is unevenly distributed over each separator plate. Hence, it is difficult for the fuel cell to generate stable electrical power for a long period of time.
The peripheral edge of each unit cell is wet-sealed with molten carbonate, whereby the reaction gases do not mix within the fuel cell. The fuel cell and an external manifold are also wet-sealed together, to prevent both reaction gases from leaking out. However, when the current density is unevenly distributed, as is described above, the fuel cell is inevitably subjected to thermal stress and subsequently becomes deformed. If this is the case, the fuel gas may leak out.